The present invention relates to an pump circuit which, for example, is applied to a semiconductor integrated circuit, such as a dynamic RAM and a flash EEPROM, and generates a voltage higher than the supply voltage in the semiconductor integrated circuit.
Recently, low power consumption in semiconductor integrated circuits has been required, and according to this demand a supply voltage has been lowered. However, there are circuits that require a voltage higher than a supply voltage in a semiconductor integrated circuit. By this reason a so-called charge pump circuit is provided for boosting the supply voltage to a predetermined voltage in the semiconductor integrated circuit, and the voltage boosted by the pump circuit is supplied to the circuit requiring a high voltage.
FIG. 29 shows an example of a conventional charge pump circuit. This charge pump circuit constituted of an inverter circuit IV to which an input signal Sin is supplied, a capacitor C as a coupling capacitor whose one end is connected to an output terminal of the inverter circuit IV, and N-channel transistors TN1, TN2 connected to the other end of the capacitor C. The inverter circuit IV is composed of a P-channel transistor TP1 and a n N-channel transistor TN3. In this circuit, the voltage of a node ND1 is transmit ted to a node ND2 via the capacitor C so as to boost the voltage of the node ND2.
FIG. 30 is waveforms showing operations of FIG. 29. The transistor TN1 is activated at a time t1, and the node ND2 is precharged to a supply voltage vcc via the transistor TN1. After this, the input voltage Sin is made a low level at a time t2. Accompanied with this, the node ND1 is made the supply voltage Vcc via the inverter circuit IV. Then, the electric potential of the node ND2 is boosted to 2 Vcc via the capacitor C. Next, at a time t3, the transistor TN2 is activated, and the electric potential of the node ND2 is output as a boosted voltage Vpp via the transistor TN2. After this at a time t4 the input voltage Sin is made a high level, and the inverter IV is inverted.
By the charge pump circuit shown in FIG. 29, a required boosted voltage Vpp can be generated. However, this circuit has a problem of a low current efficiency, that is, a high current consumption.
For example, as a method for improving the current efficiency of the charge pump circuit shown in FIG. 29, there is a pump circuit described in "An Efficient Charge Recycle and Transfer Pump Circuit for Low Operation Voltage DRAMs, Takeshi Hamamoto et al., 1996 Symposium on VLSI Circuit Digest of Technical Papers." This circuit is constituted, for example, using a plurality of charge pump circuits as shown in FIG. 29, and the improvement of the current efficiency is attempted by recycling electric charge of the capacitors of the charge pump circuits.
FIG. 31 shows a conventional two phase charge recycle pump circuit constituted using two pump circuits resembling the pump circuit described in the above mentioned literature. (In this two phase charge recycle pump circuit, electric charge is transmitted through one path from a node with a high electric potential to a node with a low electric potential.
Thus, this circuit is called two phase serial charge recycle pump circuit.) Attaching numerals 1, 2 are added to the same symbols of the same parts in FIG. 31 as those in FIG. 29. In this circuit, a transistor TN4 is connected to charge coupling nodes ND11, ND12 of capacitors C11, C12 each another. The electric charge of these nodes ND11, ND12 is recycled via the transistor TN4.
FIG. 32 shows waveforms showing operations of the circuit shown in FIG. 31. As shown in FIG. 32, in the circuit shown in FIG. 31, a P-channel transistor TP1 is turned on according to a precharge signal PRE, and the node ND11 is precharged to the supply voltage Vcc. An equalizing signal EQ is activated, and an N-channel transistor TN4 is turned on, whereby the electric potentials of the node ND11 and the node ND12 are made equal. That is, a half of the electric charge of the node ND11 is transferred to the node ND12.
With this, in the circuit shown in FIG. 31, since the electric charge of the nodes ND11, ND12 is recycled by the N-channel transistor TN4 operated according to the equalizing signal EQ, the current efficiency is improved. However, in the case of two phase charge recycle pump circuit, the fluctuations of the voltages of the nodes ND11, ND12 are decreased to 0.5 Vcc. Thus, the maximum voltage of the boosted voltage Vpp that can be output is reduced from 2 Vcc of the conventional to 1.5 Vcc.
FIG. 33 shows a conventional four phase charge recycle pump circuit (four phase serial charge recycle pump circuit) in which capacitors and transistors are further added to the circuit shown in FIG. 31, and FIG. 34 shows waveforms illustrating operations of the circuit shown in FIG. 33. In the case of four phase charge recycle pump circuit, the electric charge of the node ND11 is transferred to other nodes one after another according to the equalizing signal EQ and the precharge signal PRE. Accordingly, since the recycle frequency of the four phase charge recycle pump circuit is higher compared with the two phase charge recycle pump circuit, a utilization efficiency of current is improved so as to enable power-saving. However, in this pump circuit, the maximum voltage of the boosted voltage Vpp is reduced from 2 Vcc of the conventional to 1.25 Vcc.
In the case where the number of steps of a pump circuit is increased so as to obtain an n phase, when a maximum voltage Vpp is in vicinity of a supply voltage vcc, a maximum current efficiency is increased to a level of 1/[1+(1/n)]. However, a maximum boosted voltage is decreased to 1/[1+(1/n)]Vcc. Accordingly, there is a problem that a high voltage can not be output and efficiency is reduced in a high voltage area compared with a conventional pump circuit.
FIG. 35 shows an improved pump circuit of the circuit shown in FIG. 31. This pump circuit is a conventional two phase charge recycle pump circuit in which the electric charges charged in charge coupling nodes of two capacitors are mutually recycled. (In this two phase charge recycle pump circuit, electric charge is transmitted bidirectionally from an arbitrary node with a high electric potential to a node with a low electric potential. Thus, this circuit is called two phase parallel charge recycle pump circuit.) FIG. 36 is waveforms showing operations of FIG. 35.
In this pump circuit, the nodes ND12, ND11 are alternately precharged to a supply voltage Vcc according to precharge signals PRE1, PRE2. Then, the nodes ND11, ND12 are equalized by an N-channel transistor TN4 turned on according to the equalizing signal EQ. According to this equalizing operation the electric charges of the nodes ND11, ND12 are recycled. That is, electric charge is transferred from a node with a high electric potential to a node with a low electric potential by the operation that the nodes ND11, ND12 precharged to the supply voltage Vcc are equalized, whereby the electric charges remaining in each node ND11, ND12 are recycled. Then, current is supplied from a power supply to the node where electric potential is boosted, and the node where electric potential is lowered is grounded. Operations like this are repeated so as to generate a high voltage.
However, in each conventional charge recycle pump circuit described above, electric charge is not fully recycled. For example, in the case of the circuit shown in FIG. 35, the electric charges of the nodes ND11, ND12 are recycled only once. That is, the electric charge transferred in one recycle is the half of the electric charge remaining in each node, and the remaining 1/2 electric charge is not utilized. By this reason a large amount of current is required in order to obtain a high output voltage, thereby causing difficulty in obtaining a satisfactory current efficiency.